linux-kernel - Re: [PATCH v2 1/3] mm, printk: introduce new format string for flags

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Message-ID: <20151210025944.GB17967@js1304-P5Q-DELUXE>
Date:	Thu, 10 Dec 2015 11:59:44 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/3] mm, printk: introduce new format string for flags

Ccing, Steven to ask trace-cmd problem.

On Fri, Dec 04, 2015 at 04:16:33PM +0100, Vlastimil Babka wrote:
> In mm we use several kinds of flags bitfields that are sometimes printed for
> debugging purposes, or exported to userspace via sysfs. To make them easier to
> interpret independently on kernel version and config, we want to dump also the
> symbolic flag names. So far this has been done with repeated calls to
> pr_cont(), which is unreliable on SMP, and not usable for e.g. sysfs export.
> 
> To get a more reliable and universal solution, this patch extends printk()
> format string for pointers to handle the page flags (%pgp), gfp_flags (%pgg)
> and vma flags (%pgv). Existing users of dump_flag_names() are converted and
> simplified.
> 
> It would be possible to pass flags by value instead of pointer, but the %p
> format string for pointers already has extensions for various kernel
> structures, so it's a good fit, and the extra indirection in a non-critical
> path is negligible.

I'd like to use %pgp in tracepoint output. It works well when I do
'cat /sys/kernel/debug/tracing/trace' but not works well when I do
'./trace-cmd report'. It prints following error log.

  [page_ref:page_ref_unfreeze] bad op token &
  [page_ref:page_ref_set] bad op token &
  [page_ref:page_ref_mod_unless] bad op token &
  [page_ref:page_ref_mod_and_test] bad op token &
  [page_ref:page_ref_mod_and_return] bad op token &
  [page_ref:page_ref_mod] bad op token &
  [page_ref:page_ref_freeze] bad op token &

Following is the format I used.

TP_printk("pfn=0x%lx flags=%pgp count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
                __entry->pfn, &__entry->flags, __entry->count,
                __entry->mapcount, __entry->mapping, __entry->mt,
                __entry->val, __entry->ret)

Could it be solved by 'trace-cmd' itself?
Or it's better to pass flags by value?
Or should I use something like show_gfp_flags()?

Thanks.
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